There has already been some feed back on #3037 in #3054, so if we do an a1 release soonish, then IMO it's probably good to make the json logic customizable already.

This PR does the minimal effort solution for that, i.e. make the payload-parsing method of BaseRequest public. IMO making the raw parameters available (instead of the to_dict() versions) can be done at a later stage if there is a request for it. For now the use case should be relatively small as TGObject classes are no dataclasses yet.

Checklist for PRs

• Added .. versionadded:: version, .. versionchanged:: version or .. deprecated:: version to the docstrings for user facing changes (for methods/class descriptions, arguments and attributes)
• Created new or adapted existing unit tests
• Documented code changes according to the CSI standard
• Added myself alphabetically to AUTHORS.rst (optional)
• Added new classes & modules to the docs and all suitable __all__ s

closes #3054